problem with tomcat settings

In core Serclets and JSP by marty hall 2nd edition book it is given ,to uncomment line servlet-mapping element in web.xml file in order to enable invoker servlet.But when I do that and verify by running command catalina run from c:\tomcat\bin directory I am getting the following error. SEVERE: Parse error in default web.xmljava.lang.IllegalArgumentException: Servlet mapping specifies an unknown servlet name invoker Please Can anybody help me out? Veena